At the south end of the Forum are the confusing remains of a temple, the counterpart of the Capitolium. All that remains is the substructure, consisting of corridors and little rooms (opus reticulatum). The temple was dedicated to the goddess Roma and to the deified Emperor Augustus. It was erected during the first part of the reign of Augustus' successor, Tiberius (14-37 AD).

A priest nominated for life, the flamen Romae et Augusti, was in charge of the cult. It was a function reserved for the elite. He was supported by a temple-keeper (aedituus). From an inscription we know the name of one of these: Quintus Ostiensis Felix, a freed slave of the town.

The height of the building must have been some 16 m. It was lavishly decorated with marble (cornice, frieze, architraves, pediment, ten columns). The building provides the first example in Ostia of the extensive use of this material, and thus shows the influence of Augustan Rome. Fragments have now been attached to a modern wall to the east of the temple. In front of the same wall is a statue of Victoria, possibly flying, that may have been on the ridge of the roof of the temple. A cult statue of Roma Victrix, dressed as amazon, her left foot resting on the globe, was found in the building and is one the reasons for the identification of the temple. In the past it was displayed on top of the substructure, but it has now been taken to the museum.

The cella was approached via staircases on the sides, perhaps because the front part of the temple was used as a speaking platform by heralds, orators or judges. From an inscription we know that occasionally the city council met in the temple: "In aede Romae et Augusti placuit ordini decurionum ...", "In the temple of Roma and Augustus it pleased the order of the decurions ..." (EDR146341).

During the reign of Hadrian two brick arches were built on either side of the south end of the temple. These allowed the passage of pedestrians only.